modified
Confident
assured, sure, positive, questionless, satisfied, self-confident, undoubting, unwavering, believing, secure, untroubled, unconcerned, undisturbed, unperturbed, convinced, assertive, cocksure; see also calm 1, confident 2.
Sure
destined, determined, predestined; see inevitable.
Beyond doubt
indisputable, unquestionable, assured, positive, real, true, genuine, plain, clear, undoubted, indubitable, guaranteed, unmistakable, unassailable, sure, incontrovertible, undeniable, definite, supreme, unqualified, infallible, undisputed, unerring, sound, reliable, trustworthy, evident, conclusive, decisive, unambiguous, authoritative, irrefutable, unconditional, incontestable, unquestioned, absolute, unequivocal, inescapable, in the bag*, on ice*; see also accurate 1.
Antonyms doubtful, uncertain, dubious.
Dependable
trustworthy, safe, sound; see reliable 2.
Fixed
settled, concluded, set, agreed upon; see definite 1, determined 1.
Specific but not named
special, marked, specified, defined, one, some, a few, a couple, several, appreciable, upwards of, regular, particular, singular, especial, precise, specific, express; see also definite 1, individual 1, special 1.
certain usually suggests conviction based on specific grounds or evidence this letter makes me certain of his innocence; suresuggests merely an absence of doubt or hesitancy I'm sure you don't mean it; confident stresses the firmness of one's certainty or sureness, esp. in some expectation she's confident she'll win; positive suggests unshakable confidence, esp. in the correctness of one's opinions or conclusions, sometimes to the point of dogmatism he's too positive in his beliefs
be certain
be sure, have confidence, feel sure, be confident, have no doubt, make certain; see also know 1.
for certain